Polypropylene is a popular thermoplastic resin because it is lightweight, yet stiff, resistant to chemicals and heat, and can withstand repeated flexing. Equally important, polypropylene is less expensive than many other thermoplastics. As manufacturers look to new materials, polypropylene remains a front runner in thermoplastic applications. To improve strength, polypropylene has been reinforced with glass fibers. In automotive applications, polypropylene has been used for fan shrouds and battery cases. But, as government mandated fuel economy standards become more stringent, auto makers are looking for ways to reduce weight while simultaneously increasing the crash worthiness of their vehicles. One way companies are meeting the government and consumer mandates is by using polypropylene to replace metal and other plastics in automotive interiors and exteriors. This strategy has helped reduce both weight and cost while actually improving the recyclability of the vehicle.
Although polypropylene is known for its low cost and light weight, it has lagged behind metal and other plastic materials in terms of strength. While reinforcement with glass fibers can dramatically increase the strength and stiffness of polypropylene, its use in certain areas has been limited. Researchers have sought ways to further improve the properties of reinforced polypropylene. Despite glass fibers being an excellent source of reinforcement, a major drawback is attributed to the lack of interaction between the polar surfaces of the glass fibers with the non-polar polypropylene. The result is a very weak filler/polymer interface. After analyzing failures in composites of polypropylene and glass, it has been determined that the site of the failure is this weak interface. To combat this problem manufacturers have treated glass fibers, shortly after they are formed, with a composition, often called a size, which provides lubricating and protective properties to the fibers, and assists in providing strength and other properties to the reinforced polymers and the products made therefrom. These sizing agents help to hold the glass bundles and reduce surface polarity of the glass.
One area where reinforced polypropylene has not done as well as other plastics is in the area of automotive bumper beams. Traditionally, automotive bumpers have been made of metal, and until the low speed impact requirements were passed into law for the 1974 model year most bumpers were made of metal. Once the new standards were passed, manufacturers started looking for ways to meet these new requirements. An attractive alternative proved to be bumpers made of plastic. These new bumpers offered good protection from damage as well as having reduced weight and improved corrosion resistance.
Automotive manufacturers sought improved bumper material which is strong, light and can retain its shape even when large units are molded. Thermoplastics in the polyolefin family are a favorite with molders, especially polypropylene. However, when molded into a large piece such as a bumper beam or a fascia, polypropylene is not sufficiently stiff to withstand a 5 mph impact from a vehicle. Although polypropylene can be reinforced with glass fibers to improve its performance, even when reinforced, polypropylene falls short of meeting impact requirements demanded by auto manufacturers.